But the middle isn’t hollowing out. In fact, the ranks of Republican Senate centrists could grow as a result of November’s midterm election, following on the heels of Republican Scott Brown’s improbable election to the Senate from Massachusetts last January.

Exhibit A is Rep. Mike Castle (R) of Delaware, who is expected to win his primary next Tuesday against the "tea party" backed Christine O’Donnell and then win in the general. Congressman Castle is the quintessential moderate, both in ideology and in temperament, and knows how to work across the aisle.

Another moderate congressman who could win election to the Senate is Rep. Mark Kirk (R) of Illinois. He has had problems, to put it mildly, with the accuracy of his résumé, but his Democratic opponent, state Treasurer Alexi Giannoulias, has faced his share of questions related to the collapse of his family bank. So, perhaps suitably, the two are neck and neck in polls. If Congressman Kirk wins, he could emerge as a moderate voice in the Senate.

In fact, were Ms. McMahon to win, she would be smart to throw some early votes to the Democrats and position herself in the middle, as Brown did in liberal Massachusetts, analysts say. But given that she has never held elective office, it’s not clear how she would handle her role.

John McCain might have joined this crowd had he not veered to the right to win his Senate primary. Both he and Senator Graham, in addition to Senators Snowe and Collins, all took part in the “Gang of 14,” a bipartisan group of senators in 2005 who worked together to keep judicial nominations on track.

In a strong year for Republicans, adding moderates from the Northeast and Midwest makes sense, says former Bush White House spokesman Ari Fleischer, because those are historically where the moderates have come from.

In the 100-seat Senate, “we’re at decimated levels now, at 41, so if we’re rebuilding, those are the most likely regions,” Mr. Fleischer says.

Come January, “it’s possible that we could have a more moderate Senate, because of some of the new people, but also because of the general political dynamics that would then be in place,” says Cal Jillson, a political scientist at Southern Methodist University in Dallas.

A repeat of 1994?

The dynamic could look similar to that of the mid-1990s, when the Republicans regained control of Congress halfway through President Clinton’s first term.

“Once the Republicans have some visible responsibility for setting policy or at least helping set policy, they would be forced to be more moderate or more centrist,” Mr. Jillson says.

Another centrist voice that could join the Senate is outgoing Gov. Charlie Crist of Florida, a former Republican running as an independent. If he wins, he is likely to caucus with the Democrats. But his whole campaign is premised on getting away from partisanship.

“As an independent, I will take the best ideas of Democrats and Republicans to get things done,” he says in his first general election ad, released Tuesday.